Schmitt triggers are basic circuit blocks for both digital and analog applications. By using hysteresis, Schmitt triggers can turn a signal having a noisy or asymmetrical transition into a signal with a sharp transition region. Thus, Schmitt triggers are useful for clearing up noisy signals and to do logic level conversions. To achieve high-input impedance and relatively-low power consumption, a CMOS Schmitt trigger such as trigger 100 shown in FIG. 1 is particularly advantageous. A serial stack formed from PMOS transistors P1 and P2 and NMOS transistors N1 and N2 couple between a supply voltage VCC and ground (VSS). The gate of each transistor couples to an input voltage Vin. As will be described further, as Vin is varied with respect to a low voltage threshold and a high voltage threshold, an output voltage Vout for a node between transistors P2 and N2 will swing either to VCC or VSS. The low and high voltage thresholds may be denoted as VIL and VIH, respectively. The transition of Vout may be further understood with respect to voltages Vfp and Vfn at the sources, respectively, of a PMOS transistor P3 and an NMOS transistor N3. When transistors P3 and P1 are both on, they form a voltage divider that determines the value of Vfp according to the relative sizes of these transistors. Similarly, when transistors N3 and N1 are both on, they form a voltage divider that determines the value of Vfn according to the relative sizes of these transistors.
Should Vin be at 0 V, Vout will be at VCC. Transistor N3 will thus be on whereas transistor P3 will be off. As Vin is increased above the threshold voltage of transistor N1, N1 will be conductive. In turn, Vfn will equal a proportion of VCC as determined by the relative sizes of transistors N1 and N3 as discussed above. As Vin is further increased past Vfn plus the threshold voltage for transistor N2, N2 will start to become conductive. At this point, regenerative switching will start to occur with respect to Vout. As N2 begins to conduct, Vout will be pulled towards ground. The drop in voltage is fed back through transistor N3, which will start to turn off, thereby dropping Vfn. In turn, the dropping voltage at the drain of transistor N2 means that N2 will turn on even more robustly, thereby making Vout drop even more. In response to Vout being pulled to ground, transistor P3 will begin to turn on. The source of transistor P2 will thus be pulled low so that transistor P2 begins to turn off, causing Vout to reduce even further. In this fashion, the positive feedback through transistor N3 will rapidly pull Vout to ground. The high voltage threshold VIH for Schmitt trigger 100 will thus be approximately equal to Vfn plus the threshold voltage (VT) for transistor N2. Should transistors N1 and N3 be matched, Vfn will be approximately equal to VCC/2 such that the high voltage threshold VIH will be roughly equal to VCC/2+VT.
Now suppose Vin is gradually decreased from the high voltage threshold. As Vin drops below Vfp−VT, an analogous operation occurs through the upper portion of the stack with respect to transistors P1, P2, and P3 such that the low voltage threshold VIL equals Vfp−VT. Thus, as Vin dips below VIL, Vout will rapidly swing to VCC. The resulting relationship between Vin and Vout with respect to VIL and VIH may be seen in FIG. 2 for Schmitt trigger 100.
Because Vout will depend upon VCC, the hysteresis provided by the high and low voltages threshold will also change as VCC is changed. In modern logic systems, it is common to have a number of supply voltage levels such as 3.3 V, 2.5 V, and 1.8 V. Using the same Schmitt trigger for such a range of supply voltage levels, however, results in undesirable changes in hysteresis. For example, consider the feedback provided by PMOS transistor P3. When both transistors P3 and P1 are conducting, voltage Vfp will be approximately equal to a voltage-divided portion of VCC as discussed previously. When input voltage Vin is a threshold voltage below Vfp, transistor P2 will begin to switch on, starting the regenerative switching process that will rapidly pull Vout to VCC. But note what happens for lower levels of VCC. The threshold voltage for transistor P2 remains relatively constant such that VIL becomes closer to ground. A similar effect occurs for VIH in that it becomes closer to VCC. However, a user will typically desire a certain margin between VIL and ground and also between VIH and VCC. To satisfy a desired margin at lower values for VCC, P3 may be made relatively small with respect to P1 such the Vfp is kept closer to VCC. In turn, this makes VIL higher, thereby satisfying the desired margin. Although a reduced size for P3 thus makes operation at low VCC satisfactory, a problem will arise as higher levels of VCC are used with the same transistor size for P3. The feedback provided by such a small transistor at these higher voltages becomes proportionally less and less such that little or no hysteresis is provided. In other words, whereas the margin becomes too small unless a relatively-small transistor P3 is used at low VCC, the same transistor size produces too high of a margin at relatively-high values for VCC.
Conventional Schmitt triggers configured for use in systems having a broad range of supply voltage levels thus may include additional complex circuitry that monitors the power supply voltage level and adjusts the feedback used within the Schmitt trigger accordingly so that the desired amount of hysteresis is maintained. Discrete feedback strengths optimized for particular voltage ranges are selected by this circuitry. However, this additional circuitry occupies a relatively large circuit area which is undesirable given the general need to minimize circuit dimensions for greater density. In addition, this additional circuitry requires its own DC supply current, thereby increasing power consumption.
Accordingly, there is a need in the art for improved Schmitt triggers for operation across a range of power supply voltages.